The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Fluid delivery systems may include fluid flow meters to measure the flow rate of the fluid in the fluid delivery systems, and thus to determine the volume of the fluid to be dispensed based on the flow rate. Thermo-anemometers have been commonly used for measuring the fluid flow rate over turbine and/or paddlewheel sensors for their lack of moving parts, which are sensitive to contamination of the fluid. A thermo-anemometer operates based on the principles of heat transfer and typically includes a bridge circuit having a resistive heating element subjected to the stream of fluid flow. As the fluid flow passes over the resistive heating element, the fluid carries away heat, resulting in a temperature drop in the resistive heating element. The flow rate of the fluid can be determined by measuring the heat loss from the heating element to the fluid.
In one method, the power to the heating element is increased after a temperature drop occurs in the bridge circuit to bring the heating element back to its starting temperature (constant-temperature type). The increased power gives an indication of the fluid flow rate. Another method involves correlating the voltage drop across the bridge circuit to determine the flow rate (constant-current type).
In either method, the conventional thermo-anemometers have limitations in response time. The flow rate can not be accurately measured until the fluid flow, and hence the signals indicative of the fluid flow rate, reach a steady state. Some thermo-anemometers may require a relatively long time period to measure the flow rate of the fluid.
Another issue with the thermo-anemometers is the need for temperature compensation and part-to-part calibration. A lower temperature fluid has a greater capacity to remove heat from the resistive heating element than a higher temperature fluid at the same flow rate. Therefore, temperature compensation is generally required. Part-to-part calibration may require expensive software and may be difficult to implement in a manufacturing environment.